The future of hydrogen as an energy source is dependent upon the development of storage media with high volumetric and gravimetric capacities. Hydrogen storage has been identified as the bottleneck in the development of hydrogen-fueled vehicles. Conventional storage methods (e.g. compressed gas and liquid H2) will likely be inadequate for automotive applications due to issues of safety, volumetric H2 capacity and cost. An alternative to these more traditional methods is to store the hydrogen in the solid state. This can be accomplished with adsorbents (e.g. carbon), where hydrogen is attached to the surface of a solid, or absorbents (e.g. metal hydrides), where hydrogen is inserted in between the atoms in a solid. The key requirements for any candidate hydrogen storage material in automotive applications are high gravimetric and volumetric hydrogen densities, a release of hydrogen at moderate temperatures and pressures, and a low-cost method to recharge the material back to its original state. The US Department of Energy (US DOE) hydrogen storage system goals for the year 2010 are a 6.0 weight percent (6.0 wt %) gravimetric capacity and a volumetric capacity of 0.045 kg/L. Conventional metal hydrides that can readily supply hydrogen at room temperature have storage capacities <2 wt % and therefore cannot satisfy these goals.
Aluminum hydride, AlH3, is an attractive alternative to the traditional metal hydrides. It has a volumetric hydrogen capacity (1.48 kg/L) greater than that of liquid hydrogen and a gravimetric hydrogen capacity exceeding 10 wt %. AlH3 is stable at room temperature despite an equilibrium hydrogen pressure of around 5×105 bar at 298K [1]. In general, the rapid low temperature kinetics and high energy density make AlH3 an unusual and promising hydrogen storage medium for a number of applications.
However, the conventional organometallic synthesis is a costly procedure and AlH3 is not a reversible hydride at moderate H2 pressures. Incorporating dopants or catalytic additives is not likely to produce the large thermodynamic changes required to substantially reduce the equilibrium pressure. Therefore, the utility of this material will depend upon the development of techniques to regenerate AlH3 from the spent Al powder in a cost effective and energetically efficient manner. The present invention addresses methods to regenerate alanes (AlHx) from Al by decreasing the change in free energy during the hydrogenation reaction.
There have been a few prior attempts to hydrogenate Al to form AlHx. Baranowski and Tkacz claimed to form AlH3 from Al metal using high-pressure hydrogen (28 kbar) at 300 C. [2], however this pressure is much too low in view of the free energy of formation for AlH3 via the direct reaction of Al metal with H2 gas. Although this method is the most direct (AlH3 is formed from crystalline Al and 3/2H2), the H2 pressures required are much too large to be practical for any application (28 kbar is approximately thirty times the pressure at the bottom of the Mariana Trench, 11 km below sea level).
Birnbaum et al. [3] have made attempts to hydrogenate Al by electrochemical charging, chemical charging and by exposure to an ultrasonic field. These experiments resulted in small amounts of hydrogen uptake by the Al with concentrations of less than 2500 atomic parts per million (one quarter of 1% H in Al). These methods are costly, inefficient and do not form AlHx in high enough yields to make it a practical hydrogen fuel source.
The conventional wet chemistry procedure for synthesizing AlH3 is through an ethereal reaction of an alkali alanate (e.g. LiAlH4, NaAlH4) with aluminum chloride (AlCl3) [4] as shown in reaction (1):AlCl3+3LiAlH4+n[(C2H5)2O]→4AlH3.1.2[(C2H5)2O]+3LiCl  (1)
A non-solvated form of AlH3 was initially prepared by Chizinsky et al. [5] and subsequently by Brower et al. [6] by heating in the presence of a complex metal hydride under reduced pressure. The synthesis was extremely sensitive to the desolvation conditions (e.g. temperature and time) and small alterations lead to the precipitation of different AlH3 polymorphs. γ-AlH3 formed in the presence of excess LiAlH4, while β-AlH3 formed in the presence of excess LiAlH4 and LiBH4 [5]. In both cases, a slightly higher temperature (70 C.) and/or a longer heating time lead to the formation of α-AlH3. Although this procedure can be used to make pure AlH3, the cost of the starting materials (LiAlH4 and AlCl3) would be too high for widespread applications of AlH3 as a hydrogen storage material.
The present invention introduces new methods for regeneration of AlHx at reduced H2 gas pressures and temperatures through consideration of and adjustment for basic principles of the thermodynamics of the regeneration reaction.